The part of the electromagnetic spectrum traditional camera sensors are sensitive to ranges from ultra-violet (200-400 nm), visible light (400-700 nm) to near-infrared (700-1100 nm). While UV radiation is generally filtered out by the camera's optical elements, this is not the case for NIR. Indeed, this lack of absorption combined with the sensors' relatively high sensitivity to NIR has forced camera designers to append a specific filter, usually named “hot-mirror”, in front of the camera sensor. This is done because near-infrared is regarded as noise in the colour image formation process, as it results in “false” information in the colour channels.
By filtering out such a large part of the spectrum, however, a significant amount of potentially valuable information is lost. Moreover, near-infrared photography has existed for a long time and has been rightly popular in diverse areas such as portrait, wedding, and landscape photography, as well as astronomical imaging. The digital age has further increased consumers' interest in this form of photography, seeing that near-infrared film photography's main shortcoming was the lengthy exposure time that was required. Silicon-based sensors, however, permit exposure times that are equal to their visible light counterparts, thus making near-infrared digital photography a viable alternative.
In a typical digital camera, incident light passes through an optical focusing element (the lens), is filtered by an NIR and/or UV filter (hot-mirror) and is sampled by a colour filter array (CFA) before hitting the sensor (CCD, CMOS or other silicon-based sensor). The sensor converts incoming electromagnetic energy quanta into a voltage that is read by the camera software so as to form a raw image. This raw image is then generally further processed, in-camera, to create a full colour image, to improve the realism of colours (white balancing), to remove inherent noise, to sharpen the details, and more.
The output of a camera consists of a computer-readable file that contains three channels: red, green, and blue, akin to most displays on which images are viewed. We now focus on the physical elements that one has to replace if a high-quality near-infrared image is to be obtained in conjunction with a high-quality colour image: the hot-mirror and the CFA.
A hot-mirror is a piece of plastic or glass whose function is to filter out (by means of absorption or reflection) wavelengths that do not belong to the visible spectrum. Considered as a filter, its transmittance (exemplified in FIG. 2) is akin to a band-pass filter, as traditional sensor sensitivity to electromagnetic waves is much broader (see FIG. 3). Since radiation outside of the visible spectrum is often regarded as undesirable as the human visual system is not sensitive to it, most cameras incorporate such a hot-mirror in one form or another.
A commonly used technique to transform a standard camera into a near-infrared camera is to dismantle the camera and replace the hot-mirror by either a piece of clear glass (for focusing reasons) or an opposite filter, i.e., a filter that blocks the visible light while allowing only near-infrared light to proceed. The use of a near-infrared filter results in having a camera that cannot image the visible spectrum anymore. While the clear glass approach allows the entire range of the spectrum to be imaged, current camera designs do not allow to distinguish between the visible and near-infrared parts of the spectrum. The resulting image is therefore “wrong” and not compelling (effectively, near-infrared and visible light is merged into an RGB colour image). An alternative is to use lens-mounted filters, a hot-mirror and its opposite, to obtain two distinct image of the same scene.